EXTRA snippet of info
After drilling the 16mm hole it is best to chamfer the edge at 45 degrees. If you examine a sealant nozzle closely you will see why and then the flange will sit flat to the scuttle.

I wouldn't use a flat spade bit to drill the scuttle as it could snag in the plastic and make a right mess, I would use a proper cutter. I know you haven't drilled plastic in your test piece Techno, but anyone not sure might use spade bit which is designed to drill timber.

I have done something similar run 2 hoses fitted from drilled holes next to the wiper blade drive shafts also sealed joint in the centre of scuttle with silicon sealant and windscreen seal pulled away and smothered in silicon grease has worked well so far.

I used the same nozzle in our scuttle drain but also added a length of pipe so the water drained below the engine bay away from any electrics.
A well worth improvement taking less then 30 mins to complete.

All the posts re the water in scuttles etc prompted me to check mine over -it was sunnier today so had the bonnet up.In my Fiat 130 there is a 25mm dia. (at least) drain hole in the windscreen wiper arm well, with a drain tube that runs through the side of the wheel arch. The drain is located on the offside of the scuttle bottom.

just in case anyone else wondered if the lack of drainage encountered was for all Fiat engines

The X250 scuttle has ONE drain slightly off centre. Even with a sealed scuttle to windscreen the PROBLEM comes when parked on the opposite camber as the water builds up to sufficient height at the other end to over flow into the engine bay through the bonnet hinge slots before it goes down the provided drain.
This is mine (LHD so reversed) on level ground so ok but if I park on the wrong camber it is no good.Drain in foreground.
Fitting the improvised drain at the other end alleviates this.
Pipe goes to inner wing over wheel arch

The situation is probably worse on RHD versions as the leak will be over the fuse box.

Fun member GAVLAD obtained this extra part for me from a Citroen dealer as Fiat plead ignorance of its existence.
Very easy for me to fit as I have dispensed with my EGR pneumatic vacuum controller altogether now.
I think they intended the waste pipe to drop down the front of the engine but I found it better over the back of the gearbox
View straight up from the ground

Not only Fiat plead ignorance I contacted Peugeot in Crewe gave them the part no and they said it didn't exist even though I said I'd seen the packaged part on the internet which was clearly labelled Peugeot / Citroen part.

Looks good Andy.
I've ordered my star washers and waiting now for a decent day to fit the extra drains like yours.

On a separate note, have you ever fit DRL's to your motorhome?
LED.com have some nice ones which fit into the light recess within the bumper. - I know DRL's is a mucky word to some on here, but thought I'd ask all the same.